Diagnostic challenges in poorly differentiated primary synovial sarcoma of the breast

Synovial sarcoma (SS) is a rare and aggressive soft tissue malignancy, typically occurring in the extremities, but extremely rare as a primary tumor of the breast. We present a case of a 53-year-old Nigerian female with a recurrent breast lesion previously misdiagnosed as metaplastic carcinoma. Imaging revealed a large, multifocal mass with aggressive features. Histopathologic and immunohistochemical evaluation following modified radical mastectomy supported a diagnosis of poorly differentiated synovial sarcoma. Due to the aggressive nature and rarity of this tumor in the breast, this case highlights the diagnostic challenges and importance of molecular and immunohistochemical studies. The patient was advised to pursue chemotherapy and radiotherapy but opted to continue treatment in her home country.